province of the Roman Empire, corresponding to present-day western Hungary and parts of eastern Austria, as well as portions of several Baltic states, primarily Slovenia, Croatia, and Serbia (Vojvodina). The Pannonians were mainly Illyrians, but there were some Celts in the western part of the province. The Roman conquest of the area began in 35 BC under Octavian (who later became the emperor Augustus) and was completed in 14 BC with the capture of Sirmium (Sremska Mitrovica, Vojvodina), the key town of the Sava River valley. The Pannonian tribes, joined by the Dalmatians, revolted in AD 6, posing the gravest threat to Italy since Hannibal's invasion. After the revolt was put down, Pannonia was organized as a separate province in AD 9 and garrisoned with three legions. The emperor Trajan divided the province about AD 106. The western and northern districts constituted Pannonia Superior, which was the focal point of the Roman wars with the Marcomanni in the reign of Marcus Aurelius (reigned 161–180), who died at Vindobona (Vienna). The southern and eastern districts were organized as Pannonia Inferior under Diocletian (284–305). Pannonia Superior was divided into Pannonia Prima and Pannonia Ripariensis (or Savia), and Pannonia Inferior was divided into Valeria and Pannonia Secunda.

The inhabitants of Pannonia retained their own culture into the 2nd century AD, but Romanization did proceed rapidly, especially in the west. In the 1st century AD Emona (Ljubljana, Slovenia) and Savaria (Szombathely, Hungary) were made Roman colonies; and Scarbantia (Sopron, Hungary) and other cities were made municipia (self-governing communities). Pannonia was the birthplace of several Roman emperors of the 3rd century, and the province provided large numbers of troops for the Roman army. The grave barbarian threat in the 4th century AD forced the Romans to withdraw after 395. From that time, Pannonia ceased to exist as a separate unit.